A power converter which absorbs or releases electric power in interconnected relation to a power system is described in JP-A-8-66048. In a controller for the converter described in JP-A-8-66048, self commutated operation is performed with respect to a load by switching the control of the converter at the time of the self commutated operation of the converter from current control using a first phase-locked loop (PLL) to voltage control using a second PLL. In a converter described in Japanese Patent Application Laid-open Hei 8-9648 (JP-A-8-9648), an arrangement is provided such that when returning from self commutated operation to grid connected operation, voltage phases and amplitudes of a system voltage and a converter output are determined, and the moment when they coincide with each other, a coincidence signal is imparted to close a circuit breaker, whereupon the converter is temporarily stopped and the operation is resumed, thereby switching over control to grid connected operation.
With the conventional technique described in the aforementioned Japanese Patent Application Laid-open Hei 8-66048 (JP-A-8-66048), at the time of the switching between the self commutated operation and the grid connected operation, cases occur in which controlled variables do not utterly converge, and the controlled variables fluctuate in steps. In addition, there are cases where a converter system stops due to an overcurrent at the time of returning from the self commutated operation to the grid connected operation. For this reason, the converter must be stopped to restore the control when a changeover to the grid connected operation is effected. In addition, with the conventional technique described in the aforementioned JP-A-8-66048, there are cases where an overcurrent occurs at the time of operation of the circuit breaker.